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Re: pg_plan_advice

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Alexander Lakhin <exclusion@gmail.com>, Lukas Fittl <lukas@fittl.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2026-04-02T23:43:33Z
Lists: pgsql-hackers 
My animal sifaka just showed an all-new type of test_plan_advice
failure [1]:

diff -U3 /Users/buildfarm/bf-data/HEAD/pgsql.build/src/test/regress/expected/limit.out /Users/buildfarm/bf-data/HEAD/pgsql.build/src/test/modules/test_plan_advice/tmp_check/results/limit.out
--- /Users/buildfarm/bf-data/HEAD/pgsql.build/src/test/regress/expected/limit.out	2026-04-02 12:35:13
+++ /Users/buildfarm/bf-data/HEAD/pgsql.build/src/test/modules/test_plan_advice/tmp_check/results/limit.out	2026-04-02 12:49:59
@@ -5,6 +5,8 @@
 SELECT ''::text AS two, unique1, unique2, stringu1
 		FROM onek WHERE unique1 > 50
 		ORDER BY unique1 LIMIT 2;
+WARNING:  supplied plan advice was not enforced
+DETAIL:  advice INDEX_SCAN(onek public.onek_unique1) feedback is "matched, inapplicable, failed"
  two | unique1 | unique2 | stringu1 
 -----+---------+---------+----------
      |      51 |      76 | ZBAAAA
=== EOF ===
[12:50:02.062](11.620s) not ok 1 - regression tests pass

This is unlike the other cases we've been looking at: no sub-selects,
no GEQO, not even any joins.  There is pretty much only one plausible
plan for that query, so how did it fail?

After looking around, I think the likely explanation is that the
concurrently-run alter_table.sql test feels free to mess with the set
of indexes on onek.  It doesn't drop onek_unique1, but it does
momentarily rename it:

ALTER INDEX onek_unique1 RENAME TO attmp_onek_unique1;
ALTER INDEX attmp_onek_unique1 RENAME TO onek_unique1;

I've not looked closely at pg_plan_advice, but if it matches indexes
by name then it seems there's a window here where the advice would
fail to apply.  Also, further down we find

ALTER TABLE onek ADD CONSTRAINT onek_unique1_constraint UNIQUE (unique1);
ALTER INDEX onek_unique1_constraint RENAME TO onek_unique1_constraint_foo;
ALTER TABLE onek DROP CONSTRAINT onek_unique1_constraint_foo;

which means there's a window there where there are two plausible
indexes to choose.  Will test_plan_advice cope if the transient one
is chosen?

We could imagine dodging this problem either by having alter_table.sql
test some purpose-built table instead of a shared one, or by having it
do these hacks inside transactions so that other sessions can't see
the intermediate states.  But I'm quite resistant to that answer,
because I think part of the point here is to ensure that concurrent
DDL doesn't misbehave.  (Which it doesn't: these test fragments have
been there since 2018 and 2012 respectively, and not caused issues
AFAIK.)  Preventing our tests from exercising concurrent DDL in order
to satisfy test_plan_advice is not a good plan IMO.  There's also the
prospect of a long tail of whack-a-mole as we locate other places in
the tests where this sort of thing happens occasionally.

So I'm not sure what to do here, but we have a problem.

			regards, tom lane

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=sifaka&dt=2026-04-02%2016%3A35%3A13

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. pg_plan_advice: Fix another unique-semijoin bug.

  2. pg_plan_advice: Export feedback-related definitions.

  3. pg_plan_advice: Fix a bug when a subquery is pruned away entirely.

  4. pg_plan_advice: Add alternatives test to Makefile.

  5. pg_plan_advice: Handle non-repeatable TABLESAMPLE scans.

  6. pg_stash_advice: Allow stashed advice to be persisted to disk.

  7. Add pg_stash_advice contrib module.

  8. pg_plan_advice: Avoid assertion failure with partitionwise aggregate.

  9. pg_plan_advice: Invent DO_NOT_SCAN(relation_identifier).

  10. Add an alternative_plan_name field to PlannerInfo.

  11. pg_plan_advice: Refactor to invent pgpa_planner_info

  12. Respect disabled_nodes in fix_alternative_subplan.

  13. get_memoize_path: Don't exit quickly when PGS_NESTLOOP_PLAIN is unset.

  14. test_plan_advice: Set TAP test priority 50 in meson.build.

  15. pg_plan_advice: Avoid a crash under GEQO.

  16. Test pg_plan_advice using a new test_plan_advice module.

  17. pg_plan_advice: Always install pg_plan_advice.h, and in the right place

  18. pg_plan_advice: Fix failures to accept identifier keywords.

  19. Add pg_plan_advice contrib module.

  20. Allow extensions to mark an individual index as disabled.

  21. Replace get_relation_info_hook with build_simple_rel_hook.

  22. Store information about Append node consolidation in the final plan.

  23. Store information about elided nodes in the final plan.

  24. Store information about range-table flattening in the final plan.

  25. Pass cursorOptions to planner_setup_hook.

  26. Fix PGS_CONSIDER_NONPARTIAL interaction with Materialize nodes.

  27. Fix mistakes in commit 4020b370f214315b8c10430301898ac21658143f

  28. Allow for plugin control over path generation strategies.

  29. Update some comments for fasthash

  30. Allow passing a pointer to GetNamedDSMSegment()'s init callback.

  31. Don't reset the pathlist of partitioned joinrels.

  32. Treat number of disabled nodes in a path as a separate cost metric.